A conventional receiver receives radio frequency signals and attenuates noise from the received radio frequency signals so that a desired signal can be converted for the use of video, voice, or data. Conventional receivers are used in military, wireless, satellite communications, microwave and other technologies. From an antenna, a conventional receiver is tuned to extract the desired radio signal. The transmitted signal will have been modulated in one of several ways, for instance, amplitude modulation (AM) or frequency modulation (FM). A conventional FM receiver will now be described below with reference to FIG. 1.
FIG. 1 illustrates a conventional FM receiver 100. Receiver 100 includes an RF front end 104, a mixer 106, an FM synthesizer 108, an automatic gain controller (AGC) 118, an AGC 120, an analog to digital convertor (ADC) 122, an ADC 124 and a digital signal processor 126. Mixer 106 includes a multiplier 114 and a multiplier 116.
RF front end 104 is arranged to receive an analog signal 101 and output a conditioned RF signal 105. Mixer 106 is arranged to receive conditioned RF signal 105 and mix conditioned RF signal 105 with a frequency of interest generated by FM synthesizer 108, as will be discussed further in FIG. 2. FM synthesizer 108 generates in-phase 111 and quadrature-phase 113 clock signals of a desired frequency.
Mixer 106 multiplies conditioned RF signal 105 by I-signal 111 via multiplier 114 and outputs a mixed I-signal 115. Similarly, mixer 106 multiplies conditioned RF signal 105 by Q-signal 113 via multiplier 116 and outputs a mixed Q-signal 117.
AGC 118 and ADC 122 make up an I-signal leg of conventional receiver 100, whereas AGC 120 and ADC 124 make up a Q-signal leg of conventional receiver 100.
In particular, AGC 118 is arranged to receive and scale, i.e., amplify or attenuate as predetermined by design, mixed I-signal 115. AGC 118 is then operable to output a scaled I output signal 119. Similarly, AGC 120 is arranged to receive and scale, i.e., amplify or attenuate as predetermined by design, mixed Q-signal 113. AGC 120 is then operable to output a scaled Q output signal 121.
ADC 122 converts scaled I output signal 119 to digital signal 123. Q ADC 124 converts scaled Q output signal 121 to Q digital signal 125. I digital signal 123 and Q digital signal 125 are then received by digital signal processor 126.
In FM receiver 100, analog signal 101 is typically in the megahertz range, known as a channel, whereas the actual information to be processed by DSP 126 is in the kilohertz range. Mixer 106 down-converts analog signal 101 into the kilohertz range as mixed I-signal 115 and mixed Q-signal 117 by using a clock from a local oscillator, such that ADC 122 (and ADC 124) and DSP 126 may process the signals. In this case, the local oscillator is synthesizer 108. Synthesizer 108 is operable to generate I-signal 111 and Q-signal 113, wherein each is tunable within a predetermined band, as will be described in greater detail below. Conventionally, synthesizer 108 should be able to output signals having frequencies within a band from 65 MHz to 108 MHz, in 50 kHz steps.
Operation of synthesizer 108 will now be described in more detail with reference to FIG. 2.
FIG. 2 is a more detailed view of FM synthesizer 108 of FIG. 1. FM synthesizer 108 includes an input clock reference signal divider 202, a frequency comparator 204, an amplifier 208, a digital filter 210, a digitally controlled oscillator (DCO) 212 and an output divider 214.
Input clock reference signal divider 202 is arranged to receive an input clock reference signal 201 and output a divided reference signal 203. Frequency comparator 204 is arranged to receive I-signal 111, divided reference signal 203, channel ID signal 205 and to output a digital error signal 207. Amplifier 208 is arranged to receive digital error signal 207 and output a scaled digital error signal 209. Digital filter 210 is arranged to receive scaled digital error signal 209 and output a control signal 211. DCO 212 is arranged to receive control signal 211 and output a clock signal 213 of desired frequency. Output divider 214 is arranged to receive clock signal 213 from DCO 212 and output I-signal 111 and Q-signal 113. Each of I-signal 111 and Q-signal 113 are an integer divided version of clock signal 213. Operation of FM synthesizer 108 will now be described in more detail.
In operation, input clock reference signal divider 202 receives input clock reference signal 201. Input clock reference signal divider 202 then divides input clock reference signal 201 by a predetermined integer N in order to step down the frequency of input clock reference signal 201 and output divided reference signal 203.
Frequency comparator 204 compares I signal 111 and divided reference signal 203, with reference to channel ID signal 205. For example, if it is desired that the I-signal 111 to be 100 times the divided reference signal 203, then channel ID signal 205 may be one hundred (100), whereas if it is desired for I-signal 111 to be 50 times larger than divided reference signal 203, then channel ID signal 205 may be fifty (50), and so on.
The difference between I-signal 111 and divided reference signal 203, with reference to channel ID signal 205, is output digital error signal 207. For example, if channel ID signal 205 is one hundred (100), then digital error signal 207 will be equal to the difference between divided reference signal 203 and I-signal 111 minus one hundred, whereas if channel ID signal 205 is fifty (50) then digital error signal 207 will be equal to the difference between divided reference signal 203 and I-signal 111 minus fifty.
Amplifier 208 scales digital error signal 207 by a predetermined gain β to generate scaled digital error signal 209. Digital filter 210 then filters amplified digital error signal 209 to generate control signal 211. Control signal 211 is then fed into DCO 212 to generate output clock signal 213. Output divider 214 divides output clock signal 213 by a predetermined integer, K, as will be described in more detail below. Accordingly, output divider 214 outputs I-signal 111 and Q-signal 113, each of which is a divided-by-K version of output clock signal 213.
DCO 212 is adjusted, or tuned, to produce and maintain a predetermined relationship between input clock reference signal 201 and I-signal 111. Typically, DCO 212 is tuned by a tuning capacitor (not shown) therein. A tuning capacitor may take the form of an addressable bank of capacitors, wherein accessing an increased number of capacitors in the bank will increase the overall capacitance of the tuning capacitor and wherein accessing a decreased number of capacitors in the bank will decrease the overall capacitance of the tuning capacitor. By adjusting the capacitance of the tuning capacitor, the frequency of output clock signal 213 may be adjusted. This will now be described in more detail.
In operation, frequency comparator 204 compares the relationship between the frequency of reference divided signal 203 and the frequency of I-signal 111, with reference to channel ID signal 205. Depending on the difference, frequency comparator 204 then outputs digital error signal 207 as either a positive signal or a negative signal. A negative digital error signal 207 ultimately causes control signal 211 to increase the capacitance of the tuning capacitor within DCO 212. A positive digital error signal 207 ultimately causes control signal 211 to decrease the capacitance of the tuning capacitor within DCO 212. As the capacitance of the tuning capacitor within DCO 212 decreased or increased, DCO 212 responds by increasing or decreasing the frequency of output clock signal 213. The relationship between the frequency of divided reference signal 203 and the frequency of I-signal 111, with reference to channel ID signal 205, is again compared and the cycle repeats. In this manner, the frequency of I-signal 111 is repeatedly adjusted to eventually correspond to divided reference signal 203, with reference to channel ID signal 205. A description of operation of synthesizer 108 by way of an example will now follow.
In synthesizer 108, channel ID signal 205 is set in frequency comparator 204, integer N is set in input clock reference signal divider 202, and integer K is set in output divider 214. After receiving input clock reference signal 201, DCO 212 outputs output clock signal 213. Suppose in this example, that output clock signal 213 is too fast, i.e., the frequency of output clock signal 213 is higher than expected. Frequency comparator 203 compares I-signal 111 with divided reference signal 203, with reference to channel ID signal 205. At this point, because output clock signal 213 does not have the desired frequency as compared to input clock reference signal 201, I-signal 111 will be different from divided reference signal 203, with reference to channel ID signal 205. This difference manifests as a positive digital error signal 207.
Positive digital error signal 207 ultimately causes control signal 211 to change the capacitance of the tuning capacitor within DCO 212 to slow down output clock signal 213. Such a feed-back control loop enables output clock signal 213 to have an accurate frequency such that I-signal 111 has the appropriate relationship, based on channel ID signal 205, with to divided reference signal 203.
Once the frequency of I-signal 111 appropriately corresponds to divided reference signal 203, with reference to channel ID signal 205, synthesizer is considered to be in the “locked” state, and mainly functions to maintain I-signal 111 as “locked” to that of divided reference signal 203.
Output divider 214 receives output clock signal 213 with frequency, fclock, and produces I-signal 111 with frequency fout, where fout=fclock/K, and K is an integer.
The range of DCO 212 is a function of a tuning network therein (not shown), often comprised of a bank of capacitors. As the required frequency range of DCO 212 increases, the corresponding range of the capacitors must increase, which can lead to significant challenges due to parasitic capacitance. Specifically, multiple values of the divisor K are used in conjunction with the range of DCO 212 to represent the entire range of frequencies that can be generated by FM synthesizer 108. In general, if more values of K are supported, then the range requirement of DCO 212 is reduced.
An example tuning output of FM synthesizer 108 will now be described in more detail with reference to FIG. 3.
A typical tuning range for FM synthesizer 108 may be 65 to 110 MHz as shown in FIG. 3. The tuning range of DCO 212 may be 800 to 1000 MHz and output divider 214 is capable of dividing at integers 8, 10, 12, and 14. For an integer divider of 14, FM synthesizer 108 is capable of outputting frequencies in frequency band 302 (800 MHz/14=57.14 MHz to 1000 MHz/14=71.43 MHz). For an integer divider of 12, FM synthesizer 108 is capable of outputting frequencies in frequency band 304 (800 MHz/12=66.67 MHz to 1000 MHz/12=83.33 MHz). For an integer divider of 10, FM synthesizer 108 is capable of outputting frequencies in frequency band 306 (800 MHz/10=80 MHz to 1000 MHz/10=100 MHz). For an integer divider of 8, FM synthesizer 108 is capable of outputting frequencies in frequency band 308 (800 MHz/8=100 MHz to 1000 MHz/8=125 MHz).
FM synthesizer 108 is therefore able to output any frequency from 65 to 110 MHz using a combination of variation of DCO 212 and frequency divider 214. Output in frequency band 310 (65 to 71 MHz) may use an integer divider of 14. Output in frequency band 312 (71 to 83 MHz) may use an integer divider of 12. Output in frequency band 314 (83 to 100 MHz) may use an integer divider of 10. Output in frequency band 316 (100 to 110 MHz) may use an integer divider of 8. The maximum range of frequencies required by DCO 212 is 800 to 1000 MHz, which requires a capacitance change of about 40% when, upon a change in required output frequency of FM synthesizer 108, the output frequency of DCO 212 must change from 800 MHz to 1000 MHz or 1000 MHz to 800 MHz.
Operation of feedback output-divider 214 will now be described in more detail with reference to FIG. 4.
FIG. 4 illustrates example waveforms of output divider 214, wherein integer K dividend value of output divider 214 is eight (8) and the frequency of output clock signal 213 is one gigahertz. In such a case, I-signal 111 and Q-signal 113 will have a frequency of 1000/8, which is 125 megahertz. The figure illustrates a first waveform 402 corresponding to output clock signal 213, a second waveform 404 corresponding to output I-signal 111, and a third waveform 406 corresponding to output Q-signal 113.
In this example, as the integer K dividend value of output divider 214 is eight (8). Starting at T1, there are eight cycles in waveform 402 for one full cycle of waveform 404 ending at T4. Waveform 406 is also composed of eight cycles of waveform 302, from T3 to T5. Further, waveform 406 is delayed by an amount equal to two cycles of waveform 402, from T1 to T3. Both Q-signal 113 and I-signal 111 have a duty cycle of 50 percent when a value of eight is used for the feedback divisor because for a given set of eight clock cycles, DCO 212 is high for four clock cycles of waveform 402 and low for four clock cycles of waveform 402.
In the case where output clock signal 213 is one gigahertz (a period of one nanosecond) and the integer K dividend value of output divider 214 is eight (8), the period of waveform 404 is eight nanoseconds. A 90 degree phase shift is induced in I-signal 111 resulting in Q-signal 113 delayed by 2 nanoseconds (2 clock cycles in waveform 402) or (8 nanoseconds*90 degree/360 degree). The resulting duty cycle of I-signal 111 and Q-signal 113 is 50 percent, given that a period of I-signal 111 and Q-signal 113 is eight nanoseconds, with four nanoseconds being high and four nanoseconds being low.
A conventional method of generating I-signal 111 and Q-signal 113 is to restrict the integer K dividend value of output divider 214 to even integers. Output divider 214 is then organized to first divide fclock by K/2, to generate a frequency 2fout. This output and it's complement are divide by 2 using two separate dividers to generate I-signal 111 and Q-signal 113 in a simple manner.
In the above mentioned example, this method restricts output divider 214 to values of 8, 10, 12, or 14, where DCO 212 is one gigahertz and when divided gives the desired range of frequencies, 71 to 125 MHz (1000/8 equals 125 and 1000/14 equals ˜71.5). Thus, for a given even K value for output divider 214, DCO 212 has a tuning range of at most 20 percent. For example, when K is 8 and DCO 212 is 1000 MHz, the output of output divider 214 is 125 MHz. When K is 10 and DCO 212 is 1000 MHz, the output of output divider is 100 MHz. Therefore given a static value of K (either 8 or 10), for all required frequencies from 125 MHz to 100 MHz, DCO 212 will have to adjust up to 20%. In order to achieve such a wide tuning range, a tuning network composed of capacitors would have to be varied by up to 40 percent. This is very difficult to implement because of parasitic capacitance in DCO 212.
What is needed is a method of reducing the required range of output frequencies of DCO 212 thereby reducing or in order to reduce the required change in capacitance when adjusting DCO 212 from maximum to minimum output frequency.